1. FIELD OF THE INVENTION
The present invention relates to pistons for use in, for example, gasoline and diesel internal combustion engines and in compressors.
2. DESCRIPTION OF THE PRIOR ART
In commonly assigned U.S. Pat. No. 4,535,682 pistons are described having raised, discrete bearing surfaces formed on the piston skirt. The bearing surfaces have cooperating sloping portions which feed lubricant to those bearing surfaces and assist in generating hydrodynamic lubricant films between the bearing surfaces and the associated cylinder wall during operation of the piston in the cylinder. The effect of this is to reduce the degree of friction due to viscous drag between the piston and cylinder and which increases available power and reduces fuel consumption.
In a typical piston of the above type three discrete bearing surfaces may be provided on each of the thrust and counter-thrust portions of the skirt. There may be two bearing surfaces on the axially upper portion of the skirt below the crown and ring belt region and a single bearing surface on the axially lower skirt portion or vice versa. It has been found that when such technology as disclosed in U.S. Pat. No. 4,535,682 is applied to conventional pistons of the types known as full skirt or slipper pistons the flexibility of the lower skirt region may be greater than is desirable from the point of view of piston guidance and oil film distribution.
Strengthening or stiffening of the lower skirt region by thickening of the wall section is unacceptable due to increased weight and loss of control of that degree of flexibility which is desirable. Other methods of stiffening such as the encasting of steel inserts in the piston is again unacceptable due to both weight and production cost considerations.